The aspects of the disclosed embodiments belong to the field of aircraft flight control. More specifically, the disclosed embodiments find application during airplanes' final phases of flight, phases during which an airplane on approach to a runway is normally likely to be followed at a relatively short distance by another airplane on approach.
An airplane wing in relative movement with respect to the air generates vortices, and generally the greater the lift generated by the wing the more intense the vortex.
In practice, vortices are generated by any aerodynamic aerofoil of a finite span and an airplane wing is the source of various vortices, generally in pairs, with a vortex on a side of the wing having a symmetry with respect to a plane of symmetry of the airplane.
This phenomenon, illustrated schematically in FIG. 1 by the representations of the windings of vortex sheets attached to the wing tips, is well known and for various reasons it is beneficial to reduce the intensity of the vortices 3 of a wing 2 of an airplane 1, which is achieved, for example, by adapting the shape of the wing.
One of the reasons for which airplane manufacturers seek to reduce the intensity of the vortices generated by airplanes' wings is linked to the danger that a vortex can represent for an airplane whose path crosses a vortex caused by the passage of another airplane.
The danger in question depends upon the intensity of the vortex relative to the mass of the airplane whose path crosses the vortex. From a simple reduction in the handling comfort and the passengers' comfort when the relative intensity is low, the airplane can experience difficulties maintaining its path, or even deviate from its nominal path, when the vortex is of a moderate or high intensity, for example with disconnections of the automatic pilot, and in extreme cases the level of turbulences generated by the vortex can endanger the airplane's physical integrity.
One method, applied by all the organizations in charge of managing the circulation of airplanes in flight, consists of organizing airplanes' paths in space and time so as to maintain a sufficient distance, known as the separation distance Ds, between a following airplane located approximately on the same path as an airplane preceding it.
As a result of this separation distance, a minimum time passes before there is a risk of the following airplane penetrating the preceding airplane's vortex.
During this minimum time the vortex generated by the preceding airplane's passage has moved, displaced laterally or horizontally with respect to the path, and the speeds induced in the vortex have lessened because of the interaction of said vortex with the atmosphere so as to no longer present a risk for the following airplane.
In particular, these minimum times apply to airplanes during the approach phases, phases during which around airports the concentration of airplanes in the airspace and the necessary convergence of paths towards the landing strips are such that the issue of airplane separation is a critical problem.
The International Civil Aviation Organization (ICAO) defines the minimum separation distances that must be applied by air traffic control.
These separation distances are a function of the respective masses of the airplanes concerned. The heavier the preceding airplane and the lighter the following airplane then the greater the minimum separation distance must be, due to the greater intensity of the vortices generated by the heavy airplanes and the lighter airplanes' greater sensitivity to vortices.
For example, for what is known as an average following airplane, with a mass of between 7 and 136 tonnes, the separation distance defined by the ICAO is 3 nautical miles if the preceding airplane is a light airplane, with a mass of less than 7 tonnes, or another average airplane, and this separation distance is increased to 5 nautical miles if the preceding airplane is a heavy airplane with a mass greater than 136 tonnes.
When a light airplane follows a heavy airplane, the separation distance is increased to 6 nautical miles.
These, necessary, minimum separation distances are constraints for air traffic management and constitute a limit to the increase in an airport's capacity.
To reduce the minimum separation distances, or to not be forced to increase them in certain cases, one method consists of accelerating the natural destruction of the vortex generated by an airplane.
Patent FR 2821605, also published as U.S. Pat. No. 6,719,246, describes a device fixed on an airplane's wing that generates a periodic disruption in the flow, which perturbation provokes at least one mode of instability for the vortex that has the effect of accelerating its natural destruction.
This device thus makes it possible to reduce the minimum separation distance between two airplanes on close paths, however it requires fitting specific means on each airplane that need be determined for each model of airplane and which can pose particular installation difficulties, even more so as it is important that the specific means do not generate significant drag during the airplane's cruising phases.
Thus, while such a system can be introduced with constraints when designing a new airplane, it is more difficult to define for existing models of airplanes and requires the application of substantial modifications for airplanes in operation that were not equipped with the device to begin with.